=================== Bisecting LLVM code =================== Introduction ============ ``git bisect`` is a useful tool for finding which revision caused a bug. This document describes how to use ``git bisect``. In particular, while LLVM has a mostly linear history, it has a few merge commits that added projects -- and these merged the linear history of those projects. As a consequence, the LLVM repository has multiple roots: One "normal" root, and then one for each toplevel project that was developed out-of-tree and then merged later. As of early 2020, the only such merged project is MLIR, but flang will likely be merged in a similar way soon. Basic operation =============== See https://git-scm.com/docs/git-bisect for a good overview. In summary: .. code-block:: bash git bisect start git bisect bad main git bisect good f00ba git will check out a revision in between. Try to reproduce your problem at that revision, and run ``git bisect good`` or ``git bisect bad``. If you can't repro at the current commit (maybe the build is broken), run ``git bisect skip`` and git will pick a nearby alternate commit. (To abort a bisect, run ``git bisect reset``, and if git complains about not being able to reset, do the usual ``git checkout -f main; git reset --hard origin/main`` dance and try again). ``git bisect run`` ================== A single bisect step often requires first building clang, and then compiling a large code base with just-built clang. This can take a long time, so it's good if it can happen completely automatically. ``git bisect run`` can do this for you if you write a run script that reproduces the problem automatically. Writing the script can take 10-20 minutes, but it's almost always worth it -- you can do something else while the bisect runs (such as writing this document). Here's an example run script. It assumes that you're in ``llvm-project`` and that you have a sibling ``llvm-build-project`` build directory where you configured CMake to use Ninja. You have a file ``repro.c`` in the current directory that makes clang crash at trunk, but it worked fine at revision ``f00ba``. .. code-block:: bash # Build clang. If the build fails, `exit 125` causes this # revision to be skipped ninja -C ../llvm-build-project clang || exit 125 ../llvm-build-project/bin/clang repro.c To make sure your run script works, it's a good idea to run ``./run.sh`` by hand and tweak the script until it works, then run ``git bisect good`` or ``git bisect bad`` manually once based on the result of the script (check ``echo $?`` after your script ran), and only then run ``git bisect run ./run.sh``. Don't forget to mark your run script as executable -- ``git bisect run`` doesn't check for that, it just assumes the run script failed each time. Once your run script works, run ``git bisect run ./run.sh`` and a few hours later you'll know which commit caused the regression. (This is a very simple run script. Often, you want to use just-built clang to build a different project and then run a built executable of that project in the run script.) Bisecting across multiple roots =============================== Here's how LLVM's history currently looks: .. code-block:: none A-o-o-......-o-D-o-o-HEAD / B-o-...-o-C- ``A`` is the first commit in LLVM ever, ``97724f18c79c``. ``B`` is the first commit in MLIR, ``aed0d21a62db``. ``D`` is the merge commit that merged MLIR into the main LLVM repository, ``0f0d0ed1c78f``. ``C`` is the last commit in MLIR before it got merged, ``0f0d0ed1c78f^2``. (The ``^n`` modifier selects the n'th parent of a merge commit.) ``git bisect`` goes through all parent revisions. Due to the way MLIR was merged, at every revision at ``C`` or earlier, *only* the ``mlir/`` directory exists, and nothing else does. As of early 2020, there is no flag to ``git bisect`` to tell it to not descend into all reachable commits. Ideally, we'd want to tell it to only follow the first parent of ``D``. The best workaround is to pass a list of directories to ``git bisect``: If you know the bug is due to a change in llvm, clang, or compiler-rt, use .. code-block:: bash git bisect start -- clang llvm compiler-rt That way, the commits in ``mlir`` are never evaluated. Alternatively, ``git bisect skip aed0d21a6 aed0d21a6..0f0d0ed1c78f`` explicitly skips all commits on that branch. It takes 1.5 minutes to run on a fast machine, and makes ``git bisect log`` output unreadable. (``aed0d21a6`` is listed twice because git ranges exclude the revision listed on the left, so it needs to be ignored explicitly.) More Resources ============== https://git-scm.com/book/en/v2/Git-Tools-Revision-Selection